lets say we have a fault to earth, the earth has 25 ohms of resistance, 240v / by 25 ohms equals 9.6 amps, now tell me that is going to trip a breaker and clear a fault? and I was being VERY generous on the earth being 25 ohms, it is MUCH, MUCH higher.
It's worse than you state. The voltage between the third wire and either side of the line is only 120v. That would require less than a 4 ohm resistance to trip a 30 amp breaker--and it could likely trip only one side. I'm not comfortable with the numbers on this for shock or fire protection considering that the pump is probably out a couple hundred feet from the panel. It might be worthwhile for lightning protection of the pump.
Thats not what it says....
I read NEC 2008 section 250.56 saying that when grounding is performed by a single rod (which is the case that I mentioned), that the measured resistance be 25 ohms or less. If this value is exceeded, a second rod must be used not less than 6 feet away from the first. Unfortunately, there is no specification for the resistance of the combination. (I doubt that any inspector actually checks this, however).
Do you read 250.56 differently?
hj said:
Now wouldn't that be a recommendation from a rocket scientist, given the tendency for GFCI's to phantom trip and turn off whatever is connected to it.
Thanks for the compliment, hj--I've never worked on rockets, but have done work for the space program (NASA MDSF) and have been an IEEE member since 1974.
The reason a GFCI trips is because current is finding a path other than through the supply lines to whatever it protects. If you get consistent false trips, you either have a faulty GFCI or there's leakage to ground in the equipment that you're using.
I'll take a false trip any day to getting fried.